From the Guidelines
A normal QTc is generally considered to be less than 450 milliseconds for men and less than 460 milliseconds for women. The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization, essentially measuring the electrical activity involved in the contraction and relaxation of the ventricles. Various factors can affect the QT interval, including electrolyte abnormalities (particularly low potassium, magnesium, or calcium), certain medications, heart conditions, and genetic disorders. Regular monitoring of the QT interval is important for patients taking medications known to prolong it or for those with conditions that may affect cardiac electrical activity. According to the most recent and highest quality study, the normal upper QTc value in men is 450 ms, in women 460 ms 1. Some key points to consider when evaluating the QT interval include:
- The QT interval should be measured from the beginning of the QRS complex to the end of the T wave.
- The corrected QT interval (QTc) is more commonly used in clinical practice, which adjusts the QT interval for heart rate.
- A QTc of 0.50 second in either sex has been shown to correlate with a higher risk for torsades de pointes 1.
- Conditions known to provoke torsades de pointes, especially hypokalaemia and extreme bradycardia, should be avoided in patients with drug-induced QT prolongation 1. It is essential to note that the risk for torsades de pointes increases with the duration of the QT interval, and a QTc >500 ms is associated with a higher risk of torsades de pointes 1.
From the Research
Normal QT Interval
The normal QT interval is a crucial aspect of cardiac health, and its measurement is essential for diagnosing and managing various heart conditions.
- A normal QT interval is typically considered to be less than 440 ms for men and less than 460 ms for women 2.
- However, the normal QT interval can vary depending on age and sex, with shorter QT intervals observed in adolescent males and longer QT intervals observed in older adults 2.
- The corrected QT interval (QTc) is a more accurate measure of the QT interval, as it takes into account the heart rate 3, 2, 4, 5, 6.
- A QTc interval of greater than 500 ms is considered abnormal and may indicate an increased risk of torsades de pointes (TdP) and other life-threatening arrhythmias 3, 4.
Factors Affecting QT Interval
Several factors can affect the QT interval, including:
- Age: The QT interval tends to shorten in men during adolescence and lengthen with age 2.
- Sex: Women tend to have longer QT intervals than men, especially after puberty 2.
- Heart rate: The QT interval shortens with increasing heart rate 6.
- Autonomic tone: The QT interval can vary depending on autonomic tone and state of wakefulness, with longer QT intervals observed during sleep 6.
- Pregnancy: The QT interval may be longer in pregnant women compared to non-pregnant women, although it is still within the normal range 5.
Measurement of QT Interval
The QT interval can be measured using various methods, including: